Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
In a typical intermediate transfer type color image forming apparatus, a mode for properly setting an image density and registration (hereinafter referred to as a calibration mode) is configurable. Setting the calibration mode transfers a toner image from an image carrier to the intermediate transfer belt to form reference images (patch images) to detect toner amounts of the reference images and displaced amounts from reference positions, so as to correct print densities and color shift correction. For example, in a tandem-type-full-color-image-forming apparatus, respective image forming units of yellow, cyan, magenta, and black form reference images of the respective colors on an intermediate transfer belt, a detecting unit detects print densities and positions of the reference images, and then the print densities and color shifts are corrected.
Now, there is known a technique where two types of reference images of an oblique direction and a horizontal direction are formed, and displaced amounts in a sub-scanning direction and a main-scanning direction are detected in the calibration mode. Specifically, reference images constituted of diagonal lines B1, Y1, C1, and M1 and horizontal lines B2, Y2, C2, and M2 of respective colors of black, yellow, cyan, and magenta, which are illustrated in FIG. 7, are formed. Then, reading positions by a density detecting sensor 45a are configured to be the centers of the diagonal lines B1, Y1, C1, and M1 and the horizontal lines B2, Y2, C2, and M2 of the respective colors.
For example, when the displaced amount of the black image is detected, the displaced amount in the sub-scanning direction (a belt circumferential direction) is detected using a period b from a writing reference position w1 to a writing start position w2 of the horizontal line B2. When a formation position of the diagonal line B1 is displaced in the main-scanning direction (a belt width direction), a detecting position of the diagonal line B1 by the density detecting sensor 45a also changes. Use of this detects the displaced amount in the main-scanning direction using a difference between a period a from the writing reference position w1 to the detecting position of the diagonal line B1 and the period b. The displaced amounts of the images of yellow, cyan, and magenta are similarly detected, and then writing start positions or writing start timings of the images are adjusted on the basis of detection results.